The international agreements limiting the means of destruction of enemy combatants are contained in [Article 23 of the 1907 Hague Regulations] and in three Declarations and one Protocol, by which the contracting parties, of which Great Britain is one, engage to:

(i) “to renounce in case of war amongst themselves, the employment by their military or naval troops of any projectile of a weight below 400 grammes … which is either explosive or charged with fulminating or inflammable substances” (Declaration of St. Petersburg, 1868)

…

… This work deals only with land warfare (whether conducted by land, sea or air forces) and therefore is not concerned with air warfare. However, attention must be drawn to the Air Warfare Rules drafted at the Hague in 1923 by a commission of jurists appointed by certain Governments. Art. 18 of that code provides as follows: “The use of tracer, incendiary or explosive projectiles by or against aircraft is not prohibited.” This provision applies equally to States which are parties to the Declaration [of St. Petersburg of 1868], and those which are not. During the Second World War such projectiles were used by the air forces of all belligerents … The use of tracer and incendiary ammunition by the armed forces of belligerents was general during the Second World War and must be considered to be lawful provided that it is directed solely against inanimate military targets (including aircraft). The use of such ammunition is illegal if directed solely against combatant personnel. This is so for two reasons, first the renunciation contained in the Declaration of St. Petersburg, 1868, referred to and second the prohibition in [Article 23(e) of the 1907 Hague Regulations].

United Kingdom, The Law of War on Land being Part III of the Manual of Military Law, The War Office, HMSO, 1958, § 109 and footnote 1.

The UK LOAC Pamphlet (1981) states: “The following are prohibited in international armed conflict: a. explosive or inflammable bullets for use against personnel”.

United Kingdom, The Law of Armed Conflict, D/DAT/13/35/66, Army Code 71130 (Revised 1981), Ministry of Defence, prepared under the Direction of The Chief of the General Staff, 1981, Section 5, p. 20, § 1(a).

(emphasis in original)

The UK LOAC Manual (2004) states:

6.10. The practice of states indicates that the use of explosive or incendiary bullets designed solely for use against personnel is not permissible under customary law.

6.10.1. The reason for this is because a solid round will achieve the military purpose of disabling the enemy combatant; if a round explodes on impact it would uselessly aggravate the injury. That does not prevent the use of tracer. Nor does it prevent the use of explosive or combined-effects munitions of, for example, 0.5 or 20mm calibre for defeating materiel targets, even though personnel may be incidentally wounded by them.

6.10.2. The parties to the St Petersburg Declaration 1868 undertook “mutually to renounce, in case of war among themselves, the employment by their military or naval troops of any projectile of a weight below 400 grammes, which is either explosive or charged with fulminating or inflammable substances”. Although it is not clear from the declaration itself whether anti-materiel, as opposed to anti-personnel, uses of explosive or incendiary bullets were also contemplated, it may be inferred from state practice that it was not. The Hague Rules of Aerial Warfare 1923, provided that “the use of tracer, incendiary or explosive projectiles by or against aircraft is not prohibited”. During the Second World War such projectiles were used by the air forces of all belligerents and tracer and incendiary ammunition has since been in general use by armed forces. Incendiary weapons are not prohibited by the Conventional Weapons Convention. The 400 gram limit in the St Petersburg Declaration is, in any event, obsolete as states have developed 20mm and 25mm combined-effects munitions which weigh less than 400 grams. The use of a tracer, or small incendiary or explosive projectiles, must be considered to be lawful if it is directed against inanimate military objectives, including aircraft, or is used for range-finding or target indication. It is also lawful to use tracer mixed with normal ammunition for range-finding or target indication at night against combatant personnel, for snipers to use combined-effects munitions against either materiel or personnel targets and for aircraft to strafe enemy combatants in the open.

United Kingdom, The Manual of the Law of Armed Conflict, Ministry of Defence, 1 July 2004, §§ 6.10–6.10.2.

In its written statement submitted to the ICJ in the Nuclear Weapons casein 1995, the United Kingdom stated that the 1868 St. Petersburg Declaration prohibited projectiles the use of which “was considered to be gratuitously cruel, because it caused horrific and almost invariably fatal injuries, while offering little or no military advantage over the use of ordinary ammunition”.

United Kingdom, Written statement submitted to the ICJ, Nuclear Weapons case, 16 June 1995, § 3.65.